Neither Testicular Androgens nor Embryonic Aromatase Activity Alters Morphology of the Neural Song System in Zebra Finches1

A variety of sexual dimorphisms exist in zebra finches, both in the central nervous system and in the periphery. For example, male zebra finches sing to court females, whereas females do not normally sing. In parallel, the brain regions that control song contain a variety of male-biased sexual dimorphisms, and the vocal organ (syrinx) is larger in males than in females. It is thought that some, if not all, of these sex differences are caused by differential hormone secretion by the testes and ovaries of developing birds. However, the effects on sexual differentiation of altering the hormonal environment of birds after hatching have been ambiguous. We have found that treating embryonic female zebra finches with fadrozole, a potent aromatase inhibitor, can induce testicular tissue to develop in addition to normal ovarian tissue. This study documents the function of that testicular tissue in females by showing that it secretes androgens in adulthood, produces sperm, and causes the androgen-sensitive syrinx to enlarge. However, that functional testicular tissue has no effect on the neural song system of females; the brain regions remain completely feminine. Therefore, these results suggest that endocrinologically active testicular tissue alone is not responsible for the dramatic morphological sex differences in the zebra finch song system.